Take multi-tenancy off the roadmap

Cairn OBS is self-hosted, and the way to separate two environments is to
run two installations rather than two tenants inside one. Tenancy is the
wrong boundary for that, on three counts this repository demonstrates
rather than assumes: chwriter.WriteBatch is all-or-nothing across
tenants, so one tenant's failure stalls offset progress for every other;
CLICKHOUSE_DEFAULT_ACCESS_MANAGEMENT puts every tenant's data behind a
single superuser credential, as docker-compose.yml's own comment says;
and one binary with one set of migrations moves every tenant together,
which is the opposite of what separate environments are for. A whole
installation idles at about 1.3 GB, so the sharing buys nothing.

The project led with multi-tenant RBAC in the README banner and in
PROJECT-SPEC's goal statement. Both now say what it is instead:
self-hosted. "Open-core" goes with them -- it was already inaccurate,
since CONTRIBUTING states there is no feature gate and no paid tier, and
with enterprise/ off the roadmap there will not be one.

A second identity provider comes off the list of things standing between
this and production-ready. SSO belongs to enterprise/, and a self-hosted
deployment is not waiting on it. Terraform's tenant/RBAC resources move
from "disclosed future work" to not planned.

Nothing is scrubbed from the record. Phase 4 stays shipped, its runbook
stays, and its known gaps stay stated -- rewriting that history would
contradict the candour the Status section is built on. enterprise/ stays
in the tree, AGPLv3 and working, as the answer to a question this
project is not asking.

Signed-off-by: John Coffey <[email protected]>

terraform/README.md

@@ -188,7 +188,10 @@ Terraform state; see "`cairnobs_notification_target`'s `secret`
 attribute" above.
 
 **Also not built, all real and disclosed, not attempted here:**
-- Tenant/RBAC resources (`enterprise-auth`'s tenant/membership/grant
+- Tenant/RBAC resources -- **not planned**, since multi-tenancy came off
+  the roadmap (see the README's "Multi-tenancy is not the plan"). The
+  original accounting stands for anyone who picks it up anyway:
+  (`enterprise-auth`'s tenant/membership/grant
   surface) -- meaningfully different auth model (offline operator flags
   today, not a stable REST API a provider could safely drive
   idempotently -- see `/enterprise/README.md`'s "Bootstrapping a tenant"